In a rotary machine such as a centrifugal compressor, there is a rotary shaft of which an end protrudes to an outside of a casing to input or output a rotational force of a rotary shaft rotatably provided in the casing. In this case, it is necessary to prevent leakage of a working fluid in the casing to the outside of the casing and infiltration of foreign substances or the like into the casing from the outside thereof through a gap between the rotary shaft and a shaft insertion hole formed in the casing for the rotary shaft to pass through the casing. Therefore, a gas seal portion is provided between the rotary shaft and the casing.
The gas seal portion includes a rotary ring and a stationary ring. The rotary ring is provided integrally with the rotary shaft on an outer circumferential portion of the rotary shaft. The stationary ring is fixed to the casing and is provided to face the rotary ring in an axial direction of the rotary shaft. The stationary ring is pressed toward the rotary ring by a coil spring or the like. Therefore, in a state in which the rotary machine is stopped, the stationary ring and the rotary ring abut on each other. In addition, a spiral groove is formed on a surface of the rotary ring facing the stationary ring. When the rotary machine is operated and the rotary shaft rotates, a seal gas is introduced between the rotary ring and the stationary ring by the spiral groove. Due to a pressure of the gas, the stationary ring is pressed in the axial direction of the rotary shaft against a biasing force of the coil spring. As a result, a minute gap is formed between the rotary ring and the stationary ring. The seal gas is caused to flow from an inside of the rotary machine toward an outside thereof through the gap, and thus sealing between the rotary shaft and the casing is achieved. In this case, the pressure of the seal gas is higher than the pressure inside and outside the rotary machine.
In such a gas seal portion, the seal gas flowing from the inside of the rotary machine to the outside thereof via the gap between the rotary ring and the stationary ring is discharged to an outside through a vent (chimney) connected to the casing.
A gas or the like discharged from equipment other than the rotary machine may be delivered into the vent and may be discharged to the outside together with the seal gas. Further, depending on a type of the gas, the gas may be burned near an outlet of the vent. When the gas or the like is delivered into the vent from the equipment other than the rotary machine or the gas is burned, a pressure in the vent is increased. When the pressure in the vent becomes higher than that of the inside of the machine, the seal gas flows backward in the gap between the rotary ring and the stationary ring. Then, the rotary ring and the stationary ring may collide with each other, and thus the gas seal portion may be damaged.
Patent Document 1 discloses a constitution which includes a flow rate switch for detecting a flow rate of a gas leaking from the gas seal portion to the vent. Accordingly, when a working gas leaks due to breakage of the gas seal portion and the flow rate of the gas at the vent is increased, an abnormality is detected.
The constitution disclosed in Patent Document 1 is for detecting the breakage of the gas seal portion due to a backflow or the like of the seal gas from the vent to the gas seal portion as the abnormality. That is, it is not for preventing the breakage of the gas seal portion by suppressing a backflow of the seal gas.
Therefore, the pressure of the seal gas is usually controlled so that the pressure of the seal gas in the gas seal portion is reliably maintained at a higher level than the pressure of the vent inside and outside the rotary machine.